An oil sheen spotted in the Gulf of Mexico near Louisiana’s Grand Isle may have come from a project to plug a well in the Gulf. According to a report on NOLA.com, a second sheen spotted six miles off the coast of Grand Isle does not appear to be an oil spill at all.

Concerns were first raised about another Gulf of Mexico oil spill when oil washed ashore on Louisiana beaches on Saturday, and oil was reported on Grand Isle and other barrier islands. The U.S. Coast Guard Marine Safety Unit in Morgan City said a drilling site was being plugged when the leak occurred, according to NOLA.com.

“Samples have been taken from the shoreline impacts for testing, but the oily substance is not, at this time, suspected to be residual oil from the Deepwater Horizon oil spill,” the Coast Guard said in a press release. A company has been hired to clean up the impacted shorelines.

The Coast Guard is treating the second sheen, which extended 100 miles into the Gulf, south of Grand Isle, as a separate incident. According to the Louisiana Department of Environmental Quality, tests of the water indicated only trace amounts of petrochemicals.

According to Reuters, the Coast Guard suspects the sheen is the result of sediment carried into the Gulf by the Mississippi River. The sediment may have been released by dredging.

These new incidents come just short of the one year anniversary of the BP Gulf of Mexico oil spill. That disaster began April 20, 2010 with an explosion aboard the Deepwater Horizon oil rig that killed 11 men. Attempts to staunch the gusher failed, until a cap was successfully deployed over the undersea well on July 15. By that time, roughly 4.4 million barrels of oil had spilled into the Gulf of Mexico. The Gulf Coast economy – especially the tourism and seafood industries – is still struggling to recover from that catastrophe.